Three Unbrecciated Equilibrated Eucrites: Global Metamorphism on the Eucrite Parent Body

Details Three Unbrecciated Equilibrated Eucrites: Global Metamorphism on the Eucrite Parent Body Three Unbrecciated Equilibrated Eucrites: Global Metamorphism on the Eucrite Parent Body

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27.1469y&link_type=abstract

Lunar and Planetary Science, volume 27, page 1469

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Eucrites, Heds, History: Thermal, Metamorphism, Pyroxene

Scientific paper

We compare the petrology of two unbrecciated eucrites, EET 90020 and A-881388, with Ibitira. Unlike most eucrites, these three rocks were not significantly shocked (<1-5 Ga) after thermal metamorphism. These rocks have different textures. EET 90020 preserves a well-defined primary subophitic texture with lath-shaped plagioclase overprinted by thermal metamorphism. A 881388 shows medium-grained equigranular texture; we classified it as an eucritic granulite. Ibitira is intermediate between them; it is recrystallized but still shows the ghost of an igneous texture. The pyroxene in EET 90020 is pigeonite with a uniform distribution of fine exsolution lamellae of augite, which may have formed at high temperature (>1000 degrees C) to erase the original Ca-zoning. Ibitira and A-881388, on the other hand, contain large augite grains showing relatively high equilibration temperatures (up to 1115 degrees C). Coarse aggregates of tridymite, plagioclase and pyroxene in EET 90020 and large, elongated lathes of tridymite in EET 90020 and Ibitira might have formed by eutectic melting during metamorphism. These eucrites appear to have experienced metamorphism at high peak temperatures (presumably, just below the melting point) and prolonged slow cooling. Considering the time scale of metamorphism, these eucrites could be metamorphosed basalts formed at the time of crust formation. The very low post-metamorphic shock makes these samples ideal for age determinations.

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